The present invention relates to integrated optical circuits, which are fabricated in a photonic crystal. Specifically, a method is disclosed which creates optical devices in a photonic crystal structure using both electron beam (e-beam) and optical exposure of a resist to define an optical device surrounded by a dielectric periodic structure, a photonic crystal.
Optical circuits may be fabricated in photonic crystals, which consists of optical devices surrounded by a dielectric periodic structure exhibiting a photonic bandgap. The photonic bandgap is a range of frequencies which will not propagate through the periodic structure. The parameters of the dielectric periodic structure include the period length, refractive index of the structure, the shape of the periodic lattice as well as other factors, which determine the frequency of light that cannot propagate within the periodic structure. Light having a frequency within the photonic bandgap of the dielectric periodic structure is confined in the optical device, such as a waveguide, which constitutes a defect in the periodicity, and its further propagation is controlled by the optical device.
Processes for manufacturing such integrated optical circuits are disclosed in a published U.S. patent application (U.S. 2002/0074307A1), as well as other references. The dielectric periodic structures are formed within the dielectric layer, and an optical device such as a waveguide is formed by creating a longitudinal interruption in the periodic structure. Light having a frequency within the corresponding bandgap of the material is reflected along the internal surfaces of the longitudinal structure.
Another way of utilizing photonic crystal for waveguiding is by engineering its dispersion properties so that light is forced to propagate only along certain discrete number of directions. In this case, for arbitrary light routing, defects in the form of mirrors can be used to redirect the propagation of light from one allowed direction to another.
In either case, the usefulness of a device based on photonic crystals rests on the ability to create defects in the regular periodic array of the photonic crystal.
The creation of the periodic dielectric structure and optical components within the periodic structure typically requires the use of photo masks to define features in the integrated circuit corresponding to the periodic dielectric structure and the optical components. The use of photo masks is a significant cost factor in the manufacturing of such integrated circuits. Moreover, the creation of three-dimensional structures with devices embedded in the photonic crystal matrix requires a multilayer fabrication. In this case a working device is possible only with very precise alignment between layers, which is both costly and time consuming.
Other techniques for creating the periodic dielectric structure includes the use of optical interferometric lithography, which is useful for exposing large areas of a dielectric surface, and for creating a three-dimensional periodic dielectric structure. The process eliminates the need for a photo mask, however, not all sizes and configurations of an optical device can be formed within the periodic dielectric structure using interferometric lithography exclusively. In particular, the placement of defects, such as waveguides, resonators, or mirrors, is impractical with interferometric lithography alone.
Electron beam (e-beam) technology permits the creation of very high resolution patterning on a substrate. E-beam exposure also permits a well determined penetration depth to be obtained within a layer of resist used to form patterns of circuit components on a substrate. However, patterning large surface areas using e-beam exposure is relatively slow. Moreover, the creation of 3D patterns requires multilayer processing with all the drawbacks listed above.
It is an object of the present invention to combine optical interferometric lithography for exposing large areas of resist to create a three-dimensional periodic structure with e-beam exposure techniques to manufacture optical circuits embedded in the three-dimensional periodic structure.